Chair back



FIG. 1 is a perspective view of the chair back showing my design.

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof;

FIG. 6 is a side view thereof; and,

FIG. 7 is an opposite side view thereof.

The features presented in broken lines are for environmental purposes only and form no part of the claimed design. 

The ornamental design for a chair back, as shown and described. 